Ferroelectric random access memory (FRAM) devices are "nonvolatile" memory devices because they preserve data stored therein even in the absence of a power supply signal. Each memory cell includes a capacitor which comprises two conductive electrodes and a ferroelectric material layer formed therebetween. The ferroelectric materials used for the ferroelectric capacitor are typically Phase III potassium nitrate, bismuth titanate and lead zirconate titanate Pb(Zr, Ti)O.sub.3 (PZT). Ferroelectric materials have hysteresis characteristics. Thus, the polarity of the ferroelectric material can be maintained even after interruption of the power supply. Data (e.g., logic 0,1) is stored in the FRAM as the polarity state of the ferroelectric material in each capacitor.
The typical hysteresis characteristics of ferroelectric material will be described in detail with reference to FIG. 1. In FIG. 1, the abscissa represents a voltage V applied across the electrodes of the ferroelectric capacitor, and the ordinate represents the polarization state of the ferroelectric material. When positive and negative voltages are consecutively applied across the ferroelectric material, the polarization state of the ferroelectric material will trace a continuous loop. For example, when a predetermined positive voltage is applied across the ferroelectric material, a maximum positive polarization of +Pm at point B may be reached. When the positive voltage is removed, the degree of polarization may be reduced somewhat to +Pr at point C which denotes the residual positive polarization. Then, when a predetermined negative voltage is applied across the ferroelectric material, a maximum negative polarization of -Pm at point D may be reached. When the negative voltage is then removed, the magnitude of the degree of polarization is reduced to -Pr at point A which denotes the residual negative polarization. It is these residual polarization states which render an FRAM device as a nonvolatile device.
The polarization switching speed of a ferroelectric capacitor is approximately 10.sup.-9 sec. and the necessary program time of a ferroelectric capacitor is typically shorter than that of other nonvolatile memory devices such as electrically programmable read only memory (EPROM) devices, electrically erasable and programmable read only memory (EEPROM) devices and flash memory devices. As will be understood by those skilled in the art, the read/write cycle endurance of a ferroelectric capacitor is typically on the order of 10.sup.9 to 10.sup.12.
In conventional ferroelectric memory devices, ferroelectric-based dummy cells may be used to facilitate reading operations. The use of dummy cells is more fully described in U.S. Pat. No. 4,873,664 to Eaton, Jr. entitled "Self Restoring Ferroelectric Memory". U.S. Pat. Nos. 5,640,030, 5,373,463 and 5,608,667 also disclose ferroelectric memory devices which, in the case of the '030 patent, may contain reference cells (which are not responsive to plate line biases) to facilitate reading operations.
Unfortunately, the ferroelectric-based dummy cells contained in the memory devices of the '664 patent may undergo an exceptionally large number of repeated reading cycles and polarization reversal cycles because each dummy cell undergoes multiple cycles for every cycle a real memory cell undergoes. Thus, a reduction in the reliability of the memory devices of the '664 patent may be experienced if the cycle endurance of a dummy cell contained therein is exceeded.
Accordingly, notwithstanding the above-described nonvolatile memory devices which utilize ferroelectric materials, there continues to be a need for improved nonvolatile memory devices.